1. Field of the Invention
The present invention relates to a spectrometer having a dispersion-type spectral element such as a prism or a diffraction grating, and more particularly to a spectrometer which can measure the wavelength of sample light accurately even if the mechanism used to rotate the spectral element makes errors.
2. Description of the Related Art
Generally known in the art are spectrometers having a dispersion-type spectral element such as a prism or a diffraction grating. Of these spectrometers, one having a diffraction grating comprises other components including two plates having a slit each, two collimators, a light-receiving device, and a drive mechanism. The slit of the first plate allows the passage of sample light. The sample light passing through the first plate is converted into a parallel light beam by the first collimator. This light beam is reflected by the diffraction grating. The beam is further reflected by the second collimator and applied to the light-receiving device through the slit of the second plate. The received beam has a wavelength .lambda., which is determined by the angle at which the grating is inclined to the incident beam The light-receiving device outputs an
The drive mechanism, which includes a motor, rotates the diffraction grating, inclining the grating at a desired angle to the incident light beam. As the drive mechanism rotates the grating, the angle of inclination of the grating gradually changes, and the wavelength .lambda. of the light beam being received by the light-receiving device changes continuously. Hence, the spectrum of the sample light can be obtained by plotting the light intensities represented by the signals output by the light-receiving device.
The spectrometer described above has a drawback. As can be understood from the above, the wavelength of each light beam may not be identified by the position of the corresponding portion of the spectrum. This is because the angle of inclination of the diffraction grating is not always equal to the angle of rotation of the motor, for two reasons. First, the rotation of the motor cannot be correctly transmitted to the grating because of back-lash caused by the gears connecting the motor and the grating. Second, the motor cannot always rotate through exactly the same angle as is represented by the drive signal input to it.
Thus, the accuracy of the inclination angle of the diffraction grating depends upon the design precision of the drive mechanism and the response characteristic of assembling precision of the components.) Ultimately, the accuracy, with which the spectrometer can measure the wavelength of sample light, depends upon the design precision of the drive mechanism and the response characteristic of the motor. Unless the design precision of the mechanism or the response characteristic of the motor, or both, are improved, the wavelength-measuring accuracy of the spectrometer cannot be increased. The precision, with which the drive mechanism can rotate the diffraction grating, is about 1/30.degree. at best. This value is equivalent to a wavelength-measuring error of as much as 0.1 to 1.0 nm.